Clean-up teams at Fukushima struggled to control the melting fuel rods.
What’s the News: After the disastrous March 11 earthquake and tsunami in Japan, the world waited, mostly in vain, for details about the events that led to meltdown at the Fukushima Daiichi nuclear plant. Since then, scientists across the Pacific in California have been watching the dials of instruments that detect radioactive molecules, to see what might come across on the winds.
This week, scientists at Scripps published their readings of radioactive sulfur collected in the atmosphere in San Diego after the meltdown. These allowed them to extrapolate backwards to learn roughly how many neutrons were shed by the melting cores as workers desperately doused them in sea water, helping scientists understand the damage undergone by the cores and demonstrating the kind of remote science that may be required to help understand the events that led to meltdown.
How the Heck:
- The scientists were looking for sulfur-35, a form of sulfur that’s created when chloride ions, plentiful in sea water, are hit by neutrons produced during radioactive decay. One of the researchers told ScienceNOW that she’d read that after underwater nuclear tests in the 1950s and 60s, sulfur-35 levels had skyrocketed, and she’d suspected the same might occur after the Fukushima meltdown.
- Sure enough, in the weeks after the event, the team saw their sulfur-35 numbers, usually between 180 and 475 atoms per cubic meter of air, leap to 1500. Even this level of sulfur-35 isn’t dangerous; using a computer model, the team estimated that vast majority of radioactive sulfur molecules leaked at Fukushima—about 99.3%–likely dropped into the ocean on the way over or were otherwise dispersed.
- With their sulfur counts in hand, the team was able to back-calculate how many neutrons would have had to have hit chloride ions in order for that much sulfur to be produced. They estimate that per square meter of reactor space, 400 billions neutrons were released before March 20.
Not So Fast: While such back-calculations are an intriguing way to study the meltdown, an atmospheric scientist interviewed by ScienceNOW points out that the 6,200 miles between Japan and San Diego is a very long way for particles to travel. There must therefore be significant uncertainty in the team’s calculation of how much sulfur-35 was produced in the air above the nuclear plant.
The Future Holds: This study is likely just the harbinger of many more attempts to study the Fukushima meltdown from afar. Be on the lookout for more.
Reference: Priyadarshi, A., Dominguez, G. & Thiemens, M. H. Proc. Natl Acad. Sci. USA http://dx.doi.org/10.1073/pnas.1109449108 (2011).
Image: Wikimedia Commons / derek visser